This invention relates to well cementing, and more particularly to the cementing of wells having elevated temperatures.
In the drilling of wells, for example oil wells, wells penetrating sources of geothermal energy and the like, it is standard practice to utilize a cement to hold the well casing in position and to selectively block or plug portions of the strata through which the well extends so as to prevent the escape of undesirable fluid into the well bore or the loss of drilling muds and the like. When so used, the cement, as an aqueous slurry, is pumped into the annular space between the walls of the bore hole and the casing and permitted to cure so as to form a hardened mass which provides the reinforcing and plugging functions.
The cements utilized in drilling operations are formulated so as to be sufficiently slow setting to permit pumping and yet be sufficiently resistant to the elevated temperature and pressure conditions encountered in the walls. The American Petroleum Institute has promulgated specifications for testing cements to insure that they meet certain minimum requirements with respect to strength, permeability, settling time and the like. These cements are referred to as oil well cements.
To resist the temperatures and pressures normally encountered in wells, the oil well cements have been developed to provide the maximum physical properties under the severe temperature and pressure conditions encountered in relatively deep wells, such as oil wells. These cements are conventionally Portland-type cements to which have been added one or more various additives such as for example mica, blast furnace slag, alumina and various special reactive sands, which are designed to improve the mechanical strength and the thermal and chemical resistance of the set and hardened cement. Although satisfactory for conventional well operations, these cements have proven deficient particularly in the case of ultra-deep wells and geothermal wells where temperature in excess of 400.degree. F. (200.degree. C.) may be encountered. Under such conditions conventional cements quickly increase in porosity and lose compressive strength which may lead to a blowout. Such well blowouts are highly undesirable and can prove to be extremely dangerous, as well as costly to repair.
Accordingly, oil well cements, particularly those used in geothermal wells and in ultra deep wells, that is wells sunk to depths on the order of 25,000 feet or more, must have the ability to effectively maintain an adequate compressive strength and density and low porosity, even under high temperatures and pressures and in the presence of steam and hot brine. In addition, since the useful life of a typical well is measured in terms of 10 to 30 years, a good oil well cement must operate under the aforementioned severe conditions for a substantial period of time, preferably as long as the life of the well. However, recent studies have indicated that oil well cements presently in use have effective lives of on the order of 5 to 10 years when utilized in geothermal wells, it was noted that the strength and permeability of the oil well cement were seriously deteriorated in a period of as short as 4 years thus raising the possibility of a well blowout and increasing the frequency of replacement and maintenance.